Photoconductive elements useful in electrophotographic copiers and printers are composed of a conducting support having a photoconductive layer that is insulating in the dark but becomes conductive upon exposure to actinic radiation. To form images, the surface of the element is electrostatically and uniformly charged in the dark and then exposed to a pattern of actinic radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface and dissipate the surface charge. This leaves a charge pattern in nonirradiated areas known as a latent electrostatic image. The latent image can be developed, either on the surface on which it is formed or on another surface to which it is transferred, by application of a liquid or dry developer containing finely divided charged toner particles.
Photoconductive elements can comprise single or multiple active layers. Those with multiple active layers (also called multi-active elements) have at least one charge generation layer and at least one n-type or p-type charge generation layer. Under actinic radiation, the charge generation layer generates mobile charge carriers and the charge transport layer facilitates migration of the charge carriers to the surface of the element, where they dissipate the uniform electrostatic charge and form the latent electrostatic image.
Also useful in photoconductive elements are charge barrier layers, which are formed between the conductive support layers or a conductive smoothing layer and the charge generation layer to restrict undesired injection of charge carriers from the conductive layer. Various polymers are known for use in barrier layers of photoconductive elements.
In U.S. Pat. No. 6,294,301 B1 issued Sep. 25, 2001 to Louis J. Sorriero, Marie B. O'Regan and Michel F. Molaire (the '301 patent), discloses barrier layers comprising polyester-co-imide, polyesterionomer-co-imide, or polyamide-co-amide polymers. These units have covalently bonded as repeating units in the polymer chain, aromatic tetracarbonylbisimide groups.
While these materials have found to be effective in some instances, it has been found that such materials when used as barrier layers with the use of commonly used dispersions, particularly very fine dispersions, of materials used to form charge generation layers that during the coating process, these dispersions tend to aggregate into larger particles on the barrier layer, thus creating voids and pigment pockets that contribute to high print granularity. As a result, it has been necessary to coat thicker charge generation layers, typically from about 0.60 to about 1.0 microns, to provide acceptable granularity. A side effect of the thicker coatings is higher dark decay. High dark decay leads to reduced available toning potential.
Particularly, it has been found that pigment dispersions consisting of 1,1,2-trichloroethane solvent, a polyester ionomer and a co-crystalline mixture of titanyl phthalocyanine and titanyl fluorophthalocyanine pigment coat very nonuniformly on barrier compositions incorporating a commercial nylon polymer, Amilan CM 8000, available from Toray Industries of Japan
Accordingly, a continuing effort has been directed toward developing barrier layers that more effectively produce uniform, thin dispersion layers of the charge generation layer materials upon coating.